1. Field of the Invention
The present invention relates to a high-pressure discharge lamp.
2. Description of the Related Art
Extra-high-pressure mercury lamps are currently being used as the light source of liquid crystal projectors.
Compared to such as a metal halide lamp, a typical mercury lamp has weak light emission in the red region in the optical color rendering (spectrum distribution). Increasing the operating pressure (the internal pressure of the lamp during illumination), however, allows a continuous spectrum to be obtained in the red region even with the mercury lamp, and further, produces a light source that is superior from the viewpoints of both efficiency characteristics and life expectancy characteristics.
A high-pressure discharge lamp includes bulb 1 that is composed of: a spherical portion that forms discharge chamber 1a in the center of a glass tube; and slender glass sealing sections 1b and 1bxe2x80x2 for sealing the openings at the two ends of the glass tube, as shown in FIG. 1. In discharge chamber 1a, a pair of electrodes 4 and 4xe2x80x2 provided with cooling coils 2 and 2xe2x80x2 are arranged such that the tips of the electrodes 4 and ""4 confront each other. The back ends of these electrodes 4 and 4xe2x80x2 are connected to lead rods 7 and 7xe2x80x2, respectively, with molybdenum foil parts (Mo foil) 6 and 6xe2x80x2, respectively, interposed. The back ends of electrodes 4 and 4xe2x80x2, the molybdenum foil parts 6, and 6xe2x80x3, and one end of each of lead rods 7 and 7xe2x80x2 are hermetically buried within the glass that forms glass sealing section 1b and 1bxe2x80x2. Finally, mercury, halogen gas, and an inert gas are sealed inside discharge chamber 1a. 
However, since the operating pressure of the extra-high-pressure mercury lamp that is receiving attention as the light source of a liquid crystal projector is 200 atmospheres or more, a major problem is the prevention of damage to the lamp itself. In particular, rupture of the lamp produces a loud noise and scatters harmful substances such as mercury and halogen gas and thus poses a danger to the end user, and various measures for preventing breakage have therefore been proposed. For example, Japanese Patent Laid-Open No. 111226/1999 proposes that metal foil parts (for example molybdenum foil parts) be bonded to the electrodes that are positioned in the discharge space and that these metal foil parts be embedded in the glass that forms the sealing sections at the two ends of the lamp, the electrode side of these metal foil parts being formed in a rounded shape (curved shape).
In this official gazette, the lack of angular portions in the electrode-side ends of the metal foil parts inside the glass sealing sections provides a suppression of both concentrations of stress against these electrode-side ends and the occurrence of cracks in the electrode-side ends of the metal foil parts, whereby sufficient pressure resistance for the operating pressure can be obtained at both ends of the swelled glass portion.
Alternatively, Japanese Patent Laid-Open No. 250504/2001 proposes a construction in which the ends of electrodes and metal foil parts that are welded to these electrodes are sealed inside the sealing sections that seal the openings at the two ends of a glass tube, the welded portions of the electrodes and the metal foil parts being further covered by metal foil parts such that the ends of the electrodes are not exposed, and further, the width of the electrode-side ends of the metal foil parts being less than the width of the opposite-side ends of the electrodes. In particular, the metal foil parts are provided with a triangular shape, and the edge portions of the metal foil parts of this triangular shape are streamlined.
In this official gazette, the lack of any stepped portions between the electrodes and the metal foil parts at the welded portions of the electrodes and metal foil parts and the lack of any angles in the electrode-side ends of the metal foil parts enable a reduction of cracks that occur in the glass in the vicinity of the welded portions of the electrodes and metal foil parts when melting the two ends of the glass tube to form sealing sections, thereby obtaining an improvement in the pressure resistance of the lamp.
Japanese Patent No. 3204189 further proposes a construction in which metal foil parts (for example, molybdenum foil parts) that are bonded to electrodes that are positioned in the discharge space are buried inside the glass that forms the sealing sections at the two ends of the lamp, and further, in which coils are wrapped around the portions of the electrodes that are buried in the sealing sections.
In this official gazette, the interposition of coils between the electrodes and glass enables a reduction of the occurrence of cracks in the glass that contacts the electrode surfaces during the process of forming the sealing sections. In addition, the patent further reports that the ability to form the sealing sections at high temperature enables an improvement of the close contact between the metal foil parts and glass, whereby a lamp having sufficient pressure resistance can be provided.
Nevertheless, the measures for preventing breakage according to Japanese Patent Laid-Open No. 111226/1999 and Japanese Patent Laid-Open No. 250504/2001 focus only on the concentration of stress against the electrode-side ends of the metal foil parts inside the glass that is formed at the sealing sections, and further, the concentration of stress against the ends of the electrodes on the side of the metal foil parts. The measure for preventing damage according to Japanese Patent No. 3204189 focuses on the occurrence of cracks in the glass that contacts the electrode surface during the process of forming the sealing sections as well as on the close contact between the glass and the metal foil parts.
The primary causes for the occurrence of breakage of the lamp itself include a variety of causes in addition to those described in each of the above-described official gazettes, i.e., glass cracks that are caused by the difference in thermal expansion between the electrodes and the glass that is in contact with the electrodes during cooling following formation of the sealing sections, glass cracks that are caused by the concentration of stress against the ends of the electrodes, and glass cracks that are caused by the concentration of stress against the ends of the metal foil parts; and may also include a combination of these causes. As a consequence, the implementation of one or two of the countermeasures described in each of the official gazettes cannot be expected to have an actual effect.
Furthermore, another factor in addition to the factors described in each of the above-described official gazettes is the occurrence of a gap between the glass and the portions of the electrodes that are embedded in the glass. When such a gap is present, the high pressure that is produced inside the lamp upon lighting causes halogen gas to pass through the gap between the electrodes and the glass and bring about corrosion of the junction between the electrodes and the metal foil as well as corrosion of the metal foil, and this corrosion eventually leads to rupture of the lamp.
In the construction in which coils are wrapped around the portions of the electrodes that are embedded in the glass, as well, when an absolutely hermetic seal is not achieved between the electrodes and the coils, gaps occur between the glass and the portions of the electrodes that are embedded in the glass, and halogen gas that infiltrates this gap passes between the electrodes and the coils and brings about the above-described corrosion that leads to rupture of the lamp. Japanese Patent No. 3204189 discloses a construction in which coils are embedded only in the glass and are not exposed in the light emission space but discloses nothing regarding corrosion caused by halogen gas to the junction portion of the electrodes and metal foil as well as the metal foil itself.
In a construction in which coils are wound around the portions of the electrodes that are sealed inside the sealing sections, deformation of the metal foil that occurs when winding the coils is also a factor for shortening the life expectancy of the lamp. In other words, deformation of the metal foil reduces the close contact between the glass and metal foil, causing separation of the glass and metal foil and bringing about gas leakage of the discharge space.
It is an object of the present invention to provide a high-pressure discharge lamp that, in view of the increased operating pressure of 200 atmospheres or more, greatly reduces the causes of breakage of the lamp. To this purpose, the present invention provides a construction of a high-pressure discharge lamp that, in comparison with the prior art, can more effectively eliminate the concentration of stress and glass cracks in the vicinity of the junctions of the electrodes and metal foil parts and more effectively eliminate the effects of corrosion caused by halogen gas in the above-described vicinity of the junctions, these factors being causes for breakdown of a lamp.
The high-pressure discharge lamp of the present invention includes: a discharge chamber that is formed in a silica glass tube; a pair of electrodes each having one end that confronts the other electrode in the discharge chamber; metal foil parts that are each superposed and bonded to the other ends of the electrodes; and sealing sections for hermetically sealing the discharge chamber, these sealing sections being portions at both ends of the silica glass tube in which the other ends of the electrodes and the metal foil parts are embedded. In this high-pressure discharge lamp, the vicinities of the junctions of the electrodes and metal foil parts are buried in glass after being wrapped with metal coils. Further, the electrode-side ends of the metal foil parts are tapered. In addition, the electrode-side tips of the tapered ends are positioned, with respect to their direction of width, within the width in the radial direction of the electrodes. In this case, mercury, halogen gas, and inert gas are sealed in the discharge chamber.
According to this construction, the vicinities of the junctions of the electrodes and metal foil parts with metal coils interposed are buried in glass, thereby enabling a prevention of the occurrence of glass cracks caused by the difference in thermal expansion between the glass and the electrodes during the process of cooling after forming the sealing sections. Further, due to the tapered form of the electrodes-side ends of the metal foil parts as well as to the provision that the electrode-side tips of the tapered ends that are bonded to the ends of the electrodes be positioned, with respect to their direction of width, within the width in the radial direction of the electrodes, the metal coils can be arranged in the vicinities of the junctions of the electrodes and metal foil parts without deforming the metal foil parts, whereby the separation of glass at the metal foil parts as well as the concentration of stress in the vicinities of the junctions of the electrodes and metal foil parts can be mitigated. In addition, forming the electrode-side ends of the metal foil parts in a tapered shape and winding the metal coils as far as the ends of the electrodes can alleviate the concentration of stress at not only the ends of the metal foil parts on the side of the electrodes, but at the ends of the electrodes on the side of the metal foil parts. In other words, the construction of the present invention simultaneously solves the various causes of rupture of a lamp that were noted in the constructions of the prior art and can therefore provide a lamp that is subject to a far lower incidence of breakdown than a lamp of the prior art.
In the above-described high-pressure discharge lamp, the ends of the electrodes on the side of the metal foil parts are preferably covered by metal coils. In other words, covering the metal foil-side ends of the electrodes with metal coils provides a still greater alleviation of the concentration of stress against the metal foil-side ends of the electrodes.
Further, the dimensions of the high-pressure discharge lamp preferably satisfy the relation Wcxe2x89xa6D (more preferably, Wcxe2x89xa60.8 D) where Wc is the width of electrode-side tips of the tapered portions of the metal foil parts and D is the diameter of the electrodes; preferably satisfy the relation D/8xe2x89xa6dxe2x89xa6D/2 where d is the wire diameter of the metal coil and D is the diameter of the electrodes; preferably satisfy the relation L1xe2x89xa72D where L1 is the coil length of the metal coils and D is the diameter of the electrodes; and preferably satisfy the relation Wxe2x89xa6L2xe2x89xa63W, where L2 is the cut length of the tapered portions of the metal foil parts and W is the width of the metal foil parts.
These stipulations regarding the forms of the metal foil parts, electrodes, and metal coils enable a solution to the causes of rupture of lamps such as the glass cracks that are caused by the difference in thermal expansion between the electrodes and the glass that contacts the electrodes during the process of cooling after formation of the sealing sections, glass cracks caused by the concentration of stress against the electrode ends, glass cracks caused by the concentration of stress against the ends of the metal foil parts, and the deformation of the metal foil parts that occurs when winding coils around the portions of the electrodes that are to be embedded in glass.
In the above-described high-pressure discharge lamp, mercury is preferably injected to a level of 0.12 mg/mm3 or more; at least one of chlorine, bromine, and iodine is preferably injected as a halogen gas to a halogen gas partial pressure of 1xc3x9710xe2x88x928-1xc3x9710xe2x88x926 xcexcmol/mm3 in the discharge chamber; and the partial pressure of residual oxygen in the discharge chamber is preferably 2.5xc3x9710xe2x88x923 Pa or less. The introduction of gas in these amounts can suppress halogen gas corrosion of the junctions of the electrodes and the metal foil parts as well as corrosion of the metal foil parts despite the presence of a gap between the electrode surfaces on which the metal coils are not wrapped and the glass that surrounds these electrode surfaces of the portions of the electrodes that are embedded in the glass, and thus can effectively prevent rupture of the lamp. This construction can also prevent darkening of the glass tube and loss of luminance over long periods of illumination.
In addition, when fabricating the high-pressure discharge lamp of the present invention, the high-pressure discharge lamp is obtained by successively carrying out: a bulb formation step, an electrode assembly fabrication step, a first electrode incorporation step, a first sealing step, a mercury introduction step, a second electrode incorporation step, an evacuation step, an inert gas introduction step, a halogen gas introduction step, and a second sealing step.
A bulb having a swelled portion for the discharge chamber is first formed using a silica glass tube (Bulb Formation Step). Metal coils are next inserted onto the electrodes; the ends of the electrodes and the tapered portions of the metal foil parts are superposed; following which, either before or after the metal coils are moved and secured to the position at which the superposed portions are to be covered, the electrodes and metal foil parts are connected by welding or crimping; whereby the electrode assembly is fabricated (Electrode Assembly Preparation Step). An electrode assembly is next inserted into the opening of one end of the silica glass tube (First Electrode Incorporation Step). One end of the silica glass tube is then heated, and the other end of the electrode, the metal coil, and the metal foil parts are embedded in the glass on this end to realize a hermetic seal of the discharge chamber (First Sealing Step). Mercury is next introduced into the discharge chamber from the opening at the other end of the silica glass tube (Mercury Introduction Step), following which an electrode assembly is inserted into the opening at the other end of the silica glass tube (Second Electrode Incorporation Step). The air in the discharge chamber is then evacuated from the opening at the other end of the silica glass tube (Evacuation Step), and inert gas is introduced into the discharge chamber from the opening at this other end of the silica glass tube (Inert Gas Introduction Step). The halogen gas is next introduced into the discharge chamber from the opening at this other end of the silica glass tube (Halogen Gas Introduction Step). This end of the silica glass tube is then heated, and the other end of the electrode, the metal coil, and the metal foil parts are embedded in the glass at this end to realize a hermetic seal of the discharge chamber (Second Sealing Step).
This fabrication method can provide a high-pressure discharge lamp that, in comparison with the prior art, can reduce the concentration of stress and the glass cracking that results from this stress in the vicinities of the junctions of the electrodes and metal foil parts, and that can prevent rupture of the lamp.
In the above-described fabrication method, the residual oxygen partial pressure is preferably evacuated to 2.5xc3x9710xe2x88x923 Pa or less in the discharge chamber in the evacuation step; an amount of mercury is preferably injected to a level of at least 0.12 mg/mm3 with respect to the spatial capacity of the discharge chamber in the mercury introduction step; and halogen gas is preferably introduced such that the partial pressure of the halogen gas in the discharge chamber is within the range of 1xc3x9710xe2x88x928 to 1xc3x9710xe2x88x926 xcexcmol/mm3 in the halogen gas introduction step. This method of fabrication enables the production of a high-pressure discharge lamp that exhibits relatively little darkening of the glass tube and little drop in luminance over a long period of illumination, and moreover, that is free from corrosion by halogen gas of the junctions of the electrodes and metal foil parts as well as the metal foil parts itself.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings, which illustrate examples of the present invention.